Two similar Greenhoods compared.

Consider that you have an essay to answer in Botany Class: "Compare and contrast Diplodium obtusum and D. alveatum". That might have been your nightmare task in Botany Class, but in all probability, very few botany classes get down to that level of reality.

I shall try to make it easy for everyone. And as a Bonus I will throw in one related species which looks completely different.

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Last weekend, as you already know, the Illawarra Branch of the Australasian Native Orchid Society went on a trip into the bush near Jervis Bay. We saw many Orchids.

One of the small Greenhoods is a plant called Diplodium obtusum.
The name refers to the blunt tip of the labellum of the flower. Unfortunately the labellum is not visible in this specimen. The flower had probably been bumped by someone else taking a photo before me, and the labellum would then be "triggered" back into the closed position. David Jones says in his book that the labellum is green.

PlantNET says: "Labellum oblong, obtuse, 9–11 mm long, c. 3 mm wide green with a dark brown apex, the tip just protruding from the sinus in the set position."

Here is the Botanical illustration from PlantNET.

It is quite a small Greenhood, compared to many other members of this group of Greenhoods (Diplodium genus). Of course, there are other "Tiny Greenhoods", but they are in different genera ("Speculantha") these days.
One common feature these three plants (in today's post) all share is that they all have "stem leaves" (which are visible in two of the three images). That is part of their "Diplodium" characteristics.

Back in mid-April we (ANOS) went to Bungonia Gorge, near Marulan, NSW.
It is very different country (from Jervis Bay), with lots of slaty-shale and mixed areas of sandstone, and of course, it is close to the Bungonia caves and the South Marulan limestone quarry. So the area is renowned for its complex geology. It also has good Orchids (in season).
We saw many specimens of this other small, blunt Greenhood, which is known as Diplodium alveatum. Now I cannot say much other than this one has a labellum which is visible, and it is dark reddish brown. Apart from that, the profile of flowers of the two species is very similar.
PlantNET says: "Labellum oblong to oblong-elliptic, 7–10 mm long, 2–3 mm wide, red-brown towards the apex, the tip protruding above the sinus in the set position" You can see the Labellum here.
Here is the Botanical illustration for this species from PlantNET.
The contrasting species (of which we saw literally hundreds of flowers at Bungonia Gorge) is Diplodium laxum. Its main distinguishing feature is the "free points c. 35 mm long, filiform, usually recurved or lax." In other words, the long point which are often laid back, giving it the popular name of "Antelope Greenhood". Its labellum is reddish-grey, and protrudes by about one third of its length.
The tip of the galea is long and protruding. As the flowers age, the tip collapses over. This specimen is in very fine condition, so it has a veritable "ski-jump" angle to the point.

Here is the botanical illustration from PlantNET.

At least this one is easy to separate from the little ones with their protruding ridge or platform on the "sinus" (throat).